The present invention relates to a printing apparatus by which a photosensitive product is provided with latent images of characters, numerals, symbols marks or the like during the manufacture of the photographic product.
In the manufacture of photosensitive products such as photographic film, paper or the like (which is herein generically referred to as a "film"), it is usual to provide a film with latent images of characters, numerals, symbols, marks the like (which are herein generically referred to as "characters") which are later photographically developed during the processing of the exposed film, for providing information marks such as manufacturer's name, date, film type, frame numbers, frame size and the like along the longitudinal sides or margins of the film. One way of so marking a film, which is generally called side printing, is to optically form light images of characters on the film at the longitudinal margins adjacent each picture frame during the film manufacture, the film being thereby produced with latent images of characters, which characters are later photographically developed during the processing of the exposed film.
Apparatus for producing a film with such latent images of characters as aforementioned, which is generally called side printing apparatus, is well known as indicated by Japanese utility model No. 16589/1981, Japanese Patent unexamined publication Nos. 42037/1982 and 163226/1982 and Japanese utility model unexamined publication No. 38341/1981. Such apparatus are divided into four types in accordance with the forms of the light image providing means. One form is a projection means in which a character pattern plate and a light source for illuminating the pattern plate are used to form light images of characters. Another is a light emitting diode (LED) matrix array in which a plurality of LEDs are selectively energized to form light images of characters. The third is a liquid crystal display device in combination with light source, and the fourth is a cathode ray tube (CRT) display device.
Such side printing apparatus have been so improved that the forming of light images is caused in synchronism with the motion of the film in a forward direction by detecting the moved length of the film so as to provide latent images on the film at predetermined positions, and in that the interval for which the light image forming device is maintained energized is varied according to changes in the speed of motion of the film for the purpose of producing positive images of characters of uniform density.
One of the problems associated with the side printing apparatus above described is improper exposures characterized by double exposures on film which result in the printed characters being so over-exposed that it is hard to make them out. Such double exposures occur upon the motion of film in a reverse direction which is caused either when the tension applied to the film changes unexpectedly or when the film is suddenly stopped and there is retrogressive motion of the film transfer mechanism.